I would definitely give it a couple days. Also, as has been said repeatedly, airlock activity is no sign of fermentation. If you have a hydrometer you should take a sample to make sure that nothing is happening. If you don't have a hydrometer, get one, they're wonderful.
Finally, if you determine that your yeast are doomed, I think it would be fine to pitch another pack. Normally though, I don't think 28C (you meant celsius right? that's what I'm assuming) would completely kill the yeast, but what do I know.